Derby Talk

Derby Talk is a forum for Pinewood Derby, Awana Grand Prix, Kub Kar Rally, Shape N Race Derby, Space Derby, Raingutter Regatta and other similar races where a child and an adult work together to create a race vehicle and a lot of fun and memories

It is time to upgrade our pack scale. We want 2 scales to make inspection night go faster, so we plan on buying 2 new scales. We like having a scale that is only accurate to 0.1 oz. That way, we don't have 90 cars going back and forth trying to hit 5.00 on a scale that is accurate to 0.01 oz.

Does anybody have any recommendations on a good scale that provides only 0.1 oz accuracy? Most of the scales I see sold by the derby web sites have 0.01 oz accuracy.

Just a suggestion. We weigh our cars in grams and they must not be over 141.75. If your 1st scale is near the repair table, preferably with an adult weighing the cars, and a car is 2.6 grams over weight, you can show them a derby wheel and tell them this much weight needs to be removed. When they reach the 2nd scale they will be OK on weight. Our District race has very accurate scales and this keeps our scouts from having to work on their cars at the District level.

I think precise and precision (number of significant digits displayed) is meant in this context, rather than accurate and accuracy (how close the measurement is to the true, actual mass).

birddog wrote:That way, we don't have 90 cars going back and forth trying to hit 5.00 on a scale...

I can appreciate this logic. Would it be a satisfactory option to take a higher precision scale and mask significant digits past 0.1 ounce with, say, electrical tape?

My understanding has been that less-precise scales tend to have greater capacity and coarser sensitivity, such as postal scales, such that 5 ounces is on the lower end of the scale's range. I wonder if repeatability of marginal results might be an issue with a less-sensitive scale. For example, we have seen situations where people put their car on a postal-type scale, then add adjustable ballast onto the scale to push the weight up to 5.1 ounces, then remove a bit of the adjustable ballast to stay within 5.0 ounces (a tip a la David Meade's book, p.89). Unfortunately the car comes back with the ballast installed and the reading is 5.1 ounces anyway and another iteration is required. Perhaps people are making adjustments at a level that is beyond the sensitivity of the scale?

Unfortunately any time you use a digital scale you are subject to"the roundoff".

You must use a scale that is consistent and has enough weight resolution to create your own cutoff point.

I was asked at my race, how much over weight we allow. My answer was, 5.00oz or less. We used 3 scales of the same brand and one 5 oz official race weight. The scales were zeroed and the car was either under or over. You got to cut it off somewhere. Repeat weigh ins were tolerated to point, we may make a rule this year.

Zeroing the scale produced a very repeatable result, and I didn't have to go through the complex procedure to calibrate.

Never really thought about it, but I know where you are coming from. If somebody weigh's in at 4.92oz on a scale that resolves to 0.01, then they will go back to the workbench to install more weight. Next time it may weigh 4.96oz, so they go back again and next time maybe 4.96oz. Creates a bottleneck at the weigh in station. Whereas if the scale resolved to 0.1oz, the car would have registered 4.9oz on the first check and 5.0oz on the second.

Possible issue you could create however, is if the Pack resolves to 0.1oz, you could pass a car that weighs 5.04oz, however if they go onto district and they use a scale that resolves 0.01oz the car will be overweight as it will show as 5.04oz.

I know you are looking for a scale that resolves to 0.1oz, which this isn't, but we have been happy with the scale that Maximum Velocity sells.

While we are talking scales - I do not recommend having two scales at weigh in to expedite check-in. A helpful leader brought along an extra - identical scale. We cal'd both to the calibration weight, but just between the two locations of the weight station - we had a discrepancies in the thousandths digit as you could imagine - not what you need at checkin

Case in point: We thought we would save time by checking some cars in prior to race day, and the remainder on race day. The cars were perfect on the pre-check day. It rained the next day. The next day after was race day. The cars pre-checked were all heavy from the change in humidity. They all had to be re-weighed to be fair.

Changes in the weather, as well as different scales, cause your weight to change.

SirStorm wrote:It rained the next day. [...] The cars pre-checked were all heavy from the change in humidity.

Interesting! How do you know that the issue wasn't related to the scale?

I ask because another (perhaps simpler) hypothesis is that something happened to the scale overnight. We have noticed, for example, that temperature changes affect the timer electronics. We have also found that the (extreme) action of baking unfinished blocks to intentionally drive out moisture only causes a change of a couple percent by weight.

SirStorm wrote:But the fact still holds. You still want everyone held to the same standard, within your power.

Agreed. As mentioned in Birddog's other post related to scales, the "same standard" is promoted by occasional checking and re-calibrating as necessary using a test weight. It's not clear from an earlier comment how often your unit calibrates using a calibration weight, but a calibration weight is surely cheaper than a beam scale!

I purchased a 5 oz weight. It is THE OFFICIAL RACE WEIGHT. Master of all weight disputes. Only the judges handle this weight. We would tare on this weight then if you got a minus you were under and if you got a plus you were over.

I also purchased the appropriate calibration weight for my scale. So I can calibrate at will (200grams for this model)

My judges found the crowd walking on the gym floor and breathing on the scale messed with the readout. I left it up to them to make the call, they are judges after all. As Race Director I must delegate some things.

I really don't care what a car weighs, I just care if it is over the limit.

The participant wants to know how much, so they can add or subtract a little or alot. But for the ultimate purpose of inspection, it is a pass fail situation.

Interesting question is, does temperature and humidity effect the scales acurracy to measure or just the starting point. So, is it like a motorcycle speedometer, accurate at low speed, and lying outright at highway speeds?